The US Treasury Department has imposed sanctions on a network of businesses that provides financial support to a paramilitary force in Iran, which allegedly trains and deploys child soldiers to fight with the country's Islamic Revolutionary Guard Corps (IRGC).

The sanctions, announced in a statement on Tuesday, targets the group of at least 20 businesses and financial institutions known as the Bonyad Taavon Basij.

The Treasury said the network provides financial infrastructure to the Basij Resistance Force, a paramilitary group that works for the elite IRGC and sends child soldiers to Syria in support of President Bashar al-Assad.

Steven Mnuchin, Treasury secretary, said the international community must understand business dealings with the Bonyad Taavon Basij network has "real world humanitarian consequences".

The Basij force was involved in "violent crackdowns and serious human rights abuses" inside Iran, the Treasury said, and recruited and trained fighters, including children as young as 12.

In addition to Iranians, the Basij force also recruited Afghan immigrants as well as Pakistani nationals, the statement said.

The Basij, a paramilitary force formed soon after the 1979 revolution, is one of the Iranian regime's primary enforcers of internal security with branches in every province and city of Iran, according to the Treasury.

The Bonyad Taavon Basij is said to provide social welfare services to the Basij, including housing and financial support, and manages economic activities by funding small companies.

"Bonyad Taavon Basij has expanded its reach into Iran's economy by establishing several investment firms through its financial and investment offshoot Mehr Eqtesad Bank," the Treasury statement said.

Both Basij groups were targeted by the new sanctions.

The sanctions were imposed on Bank Mellat, Mehr Eqtesad Bank, Mehr Eqtesad Iranian Investment Co and five other investment firms, the Treasury said.

Also targeted was Iran Tractor Manufacturing Co, the Middle East's largest tractor manufacturer, and Esfahan's Mobarakeh Steel Co, the largest steelmaker in the Middle East and North Africa region, the Treasury said.

The measures come two weeks before the US President Donald Trump's administration reimposes some of the country's harshest sanctions against Iran, focusing on Iran's oil exports and financial sector.